The present invention relates generally to beam splitters and more specifically to a beam splitter for use in a liquid crystal spatial light modulator display system.
In the display industry, there is continuous pressure to produce display systems with improved image quality. This is a particular concern for display systems in which a liquid crystal spatial light modulator is illuminated by a light source to produce the viewable image. In many configurations of this type of display system, a beam splitter is used as a main component of the optics for directing light from the light source through the system. In many cases, the various surfaces of the beam splitter may cause unwanted reflections of light which pass through the system resulting in ghosting problems associated with the images produced by the display system. For illustrative purposes, a prior art system of this type will be briefly described with reference to FIG. 1.
FIG. 1 illustrates a virtual image display system indicated by reference numeral 10. System 10 includes a light source 12 and a polarizer 14 for directing polarized light into a beam splitter element 16 as indicated by arrow 18. Beam splitter element 16 is a fifty/fifty beam splitter which includes a layer of beam splitting material 20 that reflects fifty percent of the incident light and passes fifty percent of the incident light regardless of the polarization of the light. Beam splitting material 20 is supported on a transparent substrate 22.
In this example, the polarized light indicated by arrow 18 represents P-polarized light. P-polarized light 18 is directed into a reflective, liquid crystal spatial light modulator (SLM) 24. SLM 24 includes an array of pixels, each of which modulates the light by controllably changing the polarization of the light directed into each pixel, thereby forming a pattern of modulated light that is reflected back into beam splitter element 16. This pattern of modulated light from SLM 24 includes both S-polarized light and P-polarized light depending upon the light modulating states of the pixels and is indicated by arrow 26.
Beam splitter element 16 splits modulated light 26 by reflecting half of the modulated light toward an analyzer 28 as indicated by arrow 30. The other half of the modulated light is allowed to pass through beam splitting element 16 as indicated by arrow 32. Analyzer 28 blocks a particular polarization of light, in this case P-polarized light, and allows the other polarization of light, in this case S-polarized light, to pass through analyzer 28 into a viewing area indicated by viewer 34.
As illustrated in FIG. 1, system 10 forms a viewable image by using SLM 24 to modulate the polarization of the light directed into the system. However, this configuration exhibits a ghosting problem that is caused by a back surface 36 of beam splitter element 16. As described above, half of the modulated light 26 from SLM 24 is passed by beam splitter element 16 as indicated by arrow 32. As this light passes through back surface 36, a portion of the light, referred to herein as a ghosting reflection and indicated by arrow 38, is reflected by surface 36 toward the viewer 34. This ghosting reflection 38 is caused by the difference in the refractive index of the transparent substrate 22 of the beam splitter element and the air surrounding the beam splitting element. Because this ghosting reflection light 38 is slightly displaced relative to the light which was reflected by beam splitting material 20 (indicated by arrow 30), it creates a ghost image that is slightly displaced relative to the image that is intended to be produced by the system.
Depending on the material used as the transparent substrate, the ghost image may be fairly easily distinguished when a viewer views the display system. For example, using a glass substrate, the ghosting image may be in the range of about five percent of the brightness of the image that is intended to be produced by the display. A ghost image of this brightness is typically considered to be incompatible with the image quality expected of modern display systems of this type. Therefore, it is desirable to provide a more efficient beam splitter element which reduces or eliminates this ghosting problem. The present invention provides a beam splitter element that substantially eliminates this ghosting problem.